Microtubule-interacting drugs induce moderate and reversible damage to human bone marrow mesenchymal stem cells

OBJECTIVES

This study aimed to investigate molecular and cellular changes induced in human bone marrow mesenchymal stem cells (hMSCs) after treatment with microtubule-interacting agents and to estimate damage to the bone marrow microenvironment caused by chemotherapy.

MATERIALS AND METHODS

Using an in vitro hMSC culture system and biochemical and morphological approaches, we studied the effect of nocodazole and taxol(R) on microtubule and nuclear envelope organization, tubulin and p53 synthesis, cell cycle progression and proliferation and death of hMSCs isolated from healthy donors.

RESULTS AND CONCLUSIONS

Both nocodazole and taxol reduced hMSC proliferation and induced changes in the microtubular network and nuclear envelope morphology and organization. However, they exhibited only a moderate effect on cell death and partial arrest of hMSCs at G(2) but not at M phase of the cell cycle. Both agents induced expression of p53, exclusively localized in abnormally shaped nuclei, while taxol, but not nocodazole, increased synthesis of beta-tubulin isoforms. Cell growth rates and microtubule and nuclear envelope organization gradually normalized after transfer, in drug-free medium. Our data indicate that microtubule-interacting drugs reversibly inhibit proliferation of hMSCs; additionally, their cytotoxic action and effect on microtubule and nuclear envelope organization are moderate and reversible. We conclude that alterations in human bone marrow cells of patients under taxol chemotherapy are transient and reversible.

Histones H3/H4 form a tight complex with the inner nuclear membrane protein LBR and heterochromatin protein 1

We have recently shown that heterochromatin protein 1 (HP1) interacts with the nuclear envelope in an acetylation-dependent manner. Using purified components and in vitro assays, we now demonstrate that HP1 forms a quaternary complex with the inner nuclear membrane protein LBR and a sub-set of core histones. This complex involves histone H3/H4 oligomers, which mediate binding of LBR to HP1 and cross-link these two proteins that do not interact directly with each other. Consistent with previous observations, HP1 and LBR binding to core histones is strongly inhibited when H3/H4 are modified by recombinant CREB-binding protein, revealing a new mechanism for anchoring domains of under-acetylated chromatin to the inner nuclear membrane.

PBC68: a nuclear pore complex protein that associates reversibly with the mitotic spindle

Using autoimmune antibodies from a patient with primary biliary cirrhosis we have identified a 68 kDa nuclear envelope protein, termed PBC68. This protein is co-precipitated with a 98 kDa and a 250 kDa polypeptide and is distinct from the nuclear lamins. Immunostaining of digitonin-permeabilized cells indicates that PBC68 is restricted to the inner (nucleoplasmic) face of the nuclear envelope, while indirect immunofluorescence and immunoelectron microscopy show that PBC68 is located on fibrillar structures emanating from the nuclear pore complex. The autoantigen is modified at early prophase and disassembles at prometaphase concurrently with the breakdown of the nuclear envelope. The disassembled material, instead of diffusing throughout the cytoplasm as other nucleoporins, is targeted to the mitotic spindle and remains stably bound to it until anaphase. At telophase PBC68 is released from the mitotic apparatus and reassembles late, after incorporation of LAP2B and B-type lamins, onto the reforming nuclear envelope. The partitioning of PBC68 in dividing cells supports the notion that subsets of nuclear envelope proteins are actively sorted during mitosis by transiently anchoring to spindle microtubules. Furthermore, the data suggest that specific constituents of pore complex are released in a stepwise fashion from their anchorage sites before becoming available for nuclear reassembly.

Taxol affects nuclear lamina and pore complex organization and inhibits import of karyophilic proteins into the cell nucleus

Treatment of human carcinoma cells with Taxol induces focal unraveling of the nuclear lamina and extensive clustering or ectopic localization of the nuclear pore complexes. These striking aberrations develop when the cells are transferred to drug-free medium and are allowed to complete mitosis. As could be confirmed by terminal deoxynucleotidyl transferase-mediated nick end labeling assays, 4,6-diamidino-2-phenylindole staining, 5-bromo-2-deoxyuridine incorporation, and examination of the nuclear lamins by Western blotting, the malformation of the nuclear envelope is not a consequence of apoptosis or G1 arrest. In fact, Taxol-treated cells possessing a defective nuclear envelope remain alive and replication-competent for at least 24 h, undergoing programmed death 72 h after removal of the drug. While still in the nonapoptotic state, these cells lose the ability to import karyophilic proteins into the nucleus. Diminished nucleocytoplasmic transport through the nuclear pore complex can be readily demonstrated by in vitro assays involving digitonin-permeabilized cells or in vivo monitoring of nuclear factor-kappaB translocation upon stimulation with tumor necrosis factor-alpha. These observations reveal novel cellular targets of antimicrotubule drugs and may pave the way for improved schemes of anticancer treatment.